The present invention relates to an image forming apparatus.
The products known in the conventional art includes image forming apparatuses using electrophotographic process or inkjet process such as printers and multi-functional peripherals. In the image forming apparatus of this type, stability and uniformity of image quality are required. For example, in the color image forming apparatus using electrophotographic process, the color of the formed image is subject to change in conformity to changes in the environment or long-term use. For example, the Japanese Unexamined Patent Application Publication No. 2009-128885 discloses an image forming apparatus provided with a color sensor for detecting the color of the toner image subsequent to fixing operation. This image forming apparatus is equipped with an image forming section for forming a pattern image on a sheet, a pair of color sensors installed symmetrically with respect to the centerline of the sheet, which is parallel to the conveying direction, and a control section for controlling the density of the toner image according to the readings by a pair of color sensors. In this case, the image forming section forms a pattern image on the sheet positionally corresponding to the range read by one of the color sensors, but not on the sheet positionally corresponding to the range read by the other. Based on the difference of readings by a pair of color sensors, the control section controls the density of the toner image formed on the sheet.
Further, to minimize the misalignment of the image for each sheet in conformity to the misalignment (displacement in the direction perpendicular to the sheet conveyance direction) of the sheet being conveyed, and the misalignment of the image on the both sides of one sheet, the image forming apparatus uses a technique of correcting the image forming position by an image forming section (deviation correction).
Incidentally, because of the performance of the deviation correction, the image forming position of the pattern image is also corrected by the aforementioned deviation correction according to the displacement of the sheet. In this case, the positions of the pattern image detection sensor and pattern image on the sheet will be displaced, and the pattern image cannot be accurately detected by the sensor.
In view of the problems described above, it is one of the objects of the present invention to ensure accurate detection of a pattern image while minimizing the image misalignment by deviation correction.